The present invention relates to a method of analysis for electric arc discharges which occur between an inner ring of a bearing and an outer ring of the bearing.
The present invention relates further to a test stand for a roller bearing which has an inner bearing ring, an outer bearing ring, and roller elements which roll around between the inner bearing ring and the outer bearing ring.
In the case of motors which are powered by converters, parasitic effects arise which manifest themselves by a current flow through the motor bearing. As a consequence electric arc discharges can occur, through the bearing's lubricating film, which in the extreme case can lead to melting of the materials and thus to changes in the bearing race. These changes can in the long term lead to damage of the bearing, which can extend to a total failure of the bearing.
For the purpose of detecting electric arc discharges and the bearing currents, until now in the prior art measurements have only been carried out on the complete drive system, consisting of the converter and the motor. With this approach, exercising an influence over the roller bearing's operating parameters in isolation is not possible. The operating state of the roller bearing establishes itself “somehow” on the basis of the external conditions. Parameter trials, in which whole arrays of characteristic values are selectively run through, cannot be carried out in this way.
With the prior art, in order to be able to obtain an order of magnitude for the damage to the roller bearing which might be caused by the electric arc discharges, the current density of the electric arc discharges is sensed. However, this leads only to unsatisfactory correlations between the loading on the roller bearing from the electric arc discharges, on the one hand, and on the other hand the damage to the roller bearing caused thereby.
A range of parameters are responsible for the development of bearing damage. On the one hand these include, of course, the bearing current and the associated bearing voltage. On the other hand, they include further parameters, for example the mechanical and thermal operating state of the bearing. Rotational speed, mechanical vibration, temperature etc. influence the thickness of the lubricating film, and with it the probability of an electrical arc discharge. In order to be able to determine what influence the individual parameters have on the electric arc discharge, and the bearing damage associated with it, it is necessary to be able to control these parameters in isolation. Only in this way can possible dependencies, through to permissible limiting values, be determined.
A method for diagnosing journal bearings by means of magnetic field measurements is known from DE 44 41 828 A1. In the case of this method, a current is injected into the machine shaft. The leakage current through the bearing is determined from a measurement of the difference in the current which flows in the shaft before the bearing and after the bearing. The injected current is an alternating current. For the purpose of determining the currents, the magnetic fields produced by the currents are measured.
From US 2005/184 751 A1, a method is known for the detection of sparking activity in an electric machine. With this method, the shaft voltage and/or the shaft current is measured and analyzed for the purpose of sensing the sparking activity. The sparking activity can be shaft-eroding sparks which occur between parts of the shaft bearings.
From DE 44 10 639 A1, a test stand for a roller bearing is known, wherein the roller bearing has an inner bearing ring, an outer bearing ring and roller elements which roll around between the inner bearing ring and the outer bearing ring. The test stand has a base body which, on the one hand, has a mounting arrangement in which the roller bearing can be removably mounted. The test stand has in addition an electric drive, the rotational speed of which can be altered, which is arranged in a fixed location on the base body and acts on a shaft which, when the roller bearing is mounted, has a non-rotating joint to the inner bearing ring or to the outer bearing ring. With this test stand, purely mechanical testing is carried out on the roller bearing.
Under DE 198 44 822 A1, limiting values specifying the maximum permissible accelerations of a motor vehicle are stored in a non-volatile memory in a drive controller for an internal combustion engine (specifically: a car engine), as a function of the gear which is currently selected.
A first object of the present invention consists in devising a method for analyzing electric arc discharges between the inner bearing ring and the outer bearing ring of a roller bearing, which permits—unlike the prior art: reliable—predictions about bearing damage induced or not induced by the electric arc discharges.
A second object of the present invention consists in devising an appropriate test stand by means of which the above mentioned analysis method can be carried out.